The present invention relates to devices for limiting the output voltage of current-controlled remote feed apparatuses in the event of an interruption in feed circuits.
In transmitting messages through cable links in carrier frequency technology, for example in the V 300, V 960, V 2700 and V 10800 systems, the intermediate amplifiers are supplied with the necessary energy by remote means from the terminal stations through the same coaxial inner conductors through which the message signals are also transmitted by frequency multiplex. Remote power separating filters in the terminal feed stations isolate the carrier-frequency message signals from the remotely fed supply current. The remote feed itself is supplied in accordance with the so-called constant-current d.c. series feed. In this system the intermediate amplifiers are d.c. series connected and the voltage drop across the amplifier represents the operating voltage thereof. Since the amplifiers are of identical construction and the same constant current flows through each, it follows that each is supplied with the same operating voltage. Regulating devices are required only in the feeding apparatus at the terminal stations.
The remote feed voltage which occurs on the terminals of the remote feed apparatus depends on the number of intermediate amplifiers which are supplied by this voltage; in the initially-mentioned systems, this voltage reaches a maximum value between 480 and 600 V. If the remote feed circuit is interrupted, for example in the event of cable fracture or if one of the intermediate amplifiers is withdrawn, the control devices of the feed apparatus will attempt to drive the current by increasing the voltage. The consequence would be high excess voltages which could endanger the transmission equipment. Furthermore, arresters provided as lightning protection for the system would come into action and might be destroyed by the feed energy or be rendered useless for extension purposes. To avoid these effects the voltage is limited when an upper threshold is exceeded by switching an additional load resistance between the remote feed cores, the resistance being so low that the remote feed voltage is limited to a specific maximum value of approximately 60 V. The said voltage of 60 V also represents a harmless voltage if touched by persons at the position at which there is an open circuit. The additional load resistance is automatically switched off as soon as the remote feed line is restored.
Electromagnectic relays were hitherto used for switching the additional load resistance on and off (Siemens Zeitschrift 45 (1971), Nachrichtenubertragungstechnik Supplement, pp. 118-121). A total of three electromagnetic relays are required in this known arrangement.
It is an object of the invention to provide electronic means for switching the additional load resistance into and out of circuit.